What is the best way to import images as texture, then render out at original size?
I don't think there is a better way than your method, there is no direct relation between pixels and blender units, and the rendering process will always compute new values from the texture's pixel data and adjust depending on the camera's view angle.   

If you know what the label's print dimensions are ,you can set the DPI of the  image in the addon's import options, which is great since it also preserves the aspect ratio.  



If you want to load the image in the UV directly with the correct aspect ratios ,just set the display aspect ratio to the image dimensions , there is also a visual tool to check which faces are being stretched, but none to check up/down scaling sadly.